1. Field of the Invention
The present invention relates to a method and apparatus for removing unreacted monomers and unreacted lactide (dimers of lactic acid) from polylactide.
2. Background Art
For removing unreacted substances from polylactide, it has become increasingly difficult, with an increase in size of plants, to ensure the degree of vacuum that is required for removing such unreacted substances with the use of a prior-art vacuum pump alone.
In addition, during a vacuum suction time, a problem may arise that condensation products such as lactic acid monomers or lactide would scatter in an exhausting gas treatment device, which could cause plugging or corrosion.
Polylactide, which is one type of polymers synthesized by a ring-opening polymerization reaction, is polyester that is transparent and colorless and is produced from lactic acid, which is one of hydroxycarboxylic acids, as a raw material. Among methods of synthesizing polylactide from lactic acid is a method that includes the steps of producing oligomers by condensation of lactic acid, depolymerizing the oligomers by addition of a catalyst such as antimony oxide so as to produce lactide (a dimer of lactic acid), which is a cyclic condensation product, effecting ring-opening polymerization of the lactide by addition of a catalyst such as stannous octoate, and removing unreacted lactide and monomers. In such a method, pressure reduction by vacuum evacuation is performed in each process of the oligomer production, oligomer depolymerization, and monomer removal, so that unreacted monomers and lactide are removed. In that case, if the degree of vacuum is not sufficient, it would be impossible to sufficiently remove unreacted monomers and lactide, which could result in deterioration in the quality of products. The same can be said for the production of polyglycolic acid, which is synthesized by the ring-opening polymerization of glycolide which is a cyclic dimer of glycolic acid.
For removing unreacted monomers and lactide during the process of removing monomers, it is necessary to use a vacuum-evacuation removal device. The technique of Reference 1 is known as such a removal device and method.
Reference 1 (JP Patent No. 3812564 B) discloses a method of removing tetrahydrofuran, which is a by-product generated during the production of polybutylene terephthalate, which is one type of polymers obtained by polycondensation. In particular, Reference 1 discusses an exhausting gas treatment device that uses a polycondensation reaction vessel having two or more pairs of units each including an ejector, a condenser, and a hot well tank. Each ejector is driven with vapor containing 1,4-butanediol as a main component that is one of the raw materials, and the vapor containing 1,4-butanediol as a main component that is discharged from the ejector is condensed with a condenser connected downstream of each ejector, so that the polycondensation reaction vessel is put into a reduced pressure state and a polycondensation reaction is carried out.